Fat and Fat Embolism
Lipids may be classified as either “anionic” (e.g. most phospholipids), “cationic” (e.g. milk fat globules (Kiely and Olson 2003)), or “neutral”. Human fat tissue is an example of a lipid which in its living, unperturbed form, is electrically neutral. But once it is surgically or metabolically disrupted, human fat begins a breakdown into several lipid fractions, some positive, some negative, and some neutral. For example, free fatty acids (FFAs) are highly polarized molecules (de Vries et al 2004). The trans form free fatty acids (FFAs) carry a negative charge (Steinbeck et al 1991). FFAs are particularly harmful in the circulation. FFAs cause vasoconstriction and granulocytes activation through surface expression and activity of CD11b (Mastraneglo et al 1998). FFAs have been implicated in b-cell damage in the pancreas (Cnop et al 2002), tubulointerstitial damage in the kidney (Kamijo et al 2002), and acute respiratory distress syndrome in the lungs (Grotjohan et al 1996). Fortunately, because a FFA molecule is a highly polarized structure, filtration as a means to remove FFAs from the blood stream holds some promise. In fact, extracorporeal, (i.e., outside the human body) mechanical blood filtration targeting fat has been demonstrated in blood from orthopedic patients (Ramirez et al 2002). Similar extracorporeal mechanical filtration during cardiac surgery has shown that FFAs are retained by the filter particularly well, a phenomenon that is thought likely to be related to the polarity of the FFA molecule (de Vries et al 2004).
The embolization of fat particles into organs including the lung and brain is an important cause of medical morbidity, particularly following orthopedic trauma. When a bone is fractured, there is usually some fat released into the venous circulation. These particles are distributed downstream, particularly into the lung, but in most cases do not cause an obvious medical syndrome. Following orthopedic surgical procedures, however, the escaped fat particle load becomes very large, and a fat embolism syndrome may occur in a third of patients undergoing these procedures. Symptoms may range from mild respiratory distress with skin and eye symptoms, to severe pulmonary edema and death (Taviloglu et. al. 2007).
Methacrylate and Methacrylate Embolism
Methacrylate is frequently used in orthopedic surgery to affix implants and to remodel lost bone. Methyl methacrylate (MMA) polymerizes and thereby hardens into polymethyl methacrylate (PMMA). The polymer PMMA is a lipophilic molecule of varying chain length, with the molecular formula (C5O2H8)n. It is sold under a variety of medical and non-medical trade-names including the familiar “Plexiglas”. The two hydroxyl groups carry a negative electrostatic propensity, while the hydrogens impart positive charges. Consequently, the molecule has intrinsic electrostatic properties which become manifest under various polymerization and ambient pH conditions. Additionally, methacrylate molecules may be purposefully made to bear either a positive or a negative charge by means known in the art. For example, Peng et al. describe “a facile and organic-solvent-free method” involving the production of positively charged PMMA by emulsion polymerization, in which a cationic element such as the monomer methacryloyloxyethyltrimethylammonium chloride (METAC) is copolymerized with methacrylate. Likewise, negatively charged PMMA is produced using anionic comonomer sodium 2-acrylamido-2-methylpropanesulphonate (NaAMPS) (Peng et al. 2006). Particles of these materials, however, are frequently taken away from the operative site by nearby veins. When the particles are brought into the fine capillaries of the lung or other regions of the body, circulatory blockages and tissue damage may result.
Intravascular Filter Usage and Design Considerations
The engineering of vascular filters is complicated by the need to make the particle-capturing mesh tight enough to capture the targeted particles, but not so tight so as to impede circulation, or otherwise cause thrombus formation on the mesh. An excessively loose mesh (in which the spaces between the filter elements are too distant) results in failure to capture smaller emboli. Conversely, a mesh that is too tight (in which filter elements are too close to one another) increases the resistance to blood flow, and may trap particles indiscriminately, leading to early thrombosis and occlusion of paths through the filter.
Electrostatic Filters
Electrostatic filters are known principally for use in water filtration, cleaning of fabric, air/allergen filtration, and in food processing, but have not been adapted to the unique environment and demands of intravascular use.
It would be desirable to have a venous filter capable of capturing small embolic particles, including the most dangerous fatty acids, without attracting platelets and promoting thrombosis. It would also be desirable to deploy such a filter via a catheter prior to a high-embolic-risk procedure, and to be able to retrieve it at the conclusion of that procedure.
The invention set forth herein relates to a retrievable protective mesh which is inserted into a blood vessel which is deemed at risk for delivering potentially harmful embolic particles to distal organs. This mesh is deployed via catheter, or by direct cut-down into the vein, and is of sufficient patency to allow normal blood cells and small clumps of cellular material through. In particular, this intravascular filter is constructed to employ electrostatic forces in a manner that permits adhesive forces to capture particles of the targeted type (for example fat or methacrylate emboli), and to retain these particles, even those which might otherwise be physically small enough to slip through the filter. Specific types of targeted particles are thereby captured and retained with improved efficiency, permitting filter elements to be more widely spaced than would otherwise be necessary, thereby decreasing both resistance and the propensity for thrombosis. The device is designed to be retrieved post-op, and the accumulated debris on the mesh analyzed in the laboratory. The device provides protection from embolism and stroke resulting from debris released by sites of tissue trauma. The device provides protection from embolism and stroke resulting from debris released by sites of tissue trauma.